1. Field of the Invention
The invention relates to multi-pass ink-jet printing. More particularly, the invention relates to a method and system for detecting one or more failed ink jets and thereafter compensating for the one or more failed ink jets with the remaining operational ink jets.
2. Description of the Related Technology
Multi-pass printing is a technique used to reduce banding in ink-jet printing. Dots of ink, when still in liquid form, tend to run together due to surface tension. This is referred to as coalescence. To print a high quality image it is important to print individual round dots. But to achieve full saturated colors, the dots must overlap to completely cover the paper. By only printing a portion of the image data so as to avoid simultaneously printing adjacent dots during each printing cycle, coalescence may be largely avoided. Additionally, by avoiding all horizontal adjacencies, the transverse speed of the printing mechanism can be increased up to two times the rated print speed of the printhead.
Multi-pass printing is accomplished by filtering the image data using a print mask to determine which dots are to be printed in each swath. A swath is defined as a region, or portion, of a recording medium which is printed upon by a given portion, or print zone, of a printhead cartridge having a specified number of ink jets, as the printhead cartridge passes over the recording medium. The swath successively advances through each print zone after each pass of the printhead and is partially printed in each print zone. The printing of a swath is completed after it has successively advanced through each print zone.
In multi-pass printing, each jet of a printhead is assigned the role of ejecting ink, as necessary, onto pre-specified areas or "dots" on a raster line which is currently in the jet's respective print zone. However, a jet sometimes fails either due to being clogged or electrical problems in its firing circuitry. When this occurs, the pre-specified areas which are assigned to the failed jet in accordance with a respective jet mask, are not printed upon. Therefore, if one or more jets fail and there are many areas in which intended ink drops are not deposited, the quality of the printed image may significantly suffer.
Prior art methods have dealt with this problem by utilizing auxiliary jets in the printhead which are assigned the task of replacing failed jets. However, this method is inefficient because these auxiliary jets are inactive during periods when there are no jet failures. Therefore, the auxiliary jets represent printhead resources which are not fully utilized to their maximum potential. Additionally, in order to safeguard against situations in which multiple jets simultaneously fail, not just one but a bank of multiple auxiliary jets are set aside in the printhead for taking the place of failed jets. Although printheads having multiple auxiliary printheads improve the reliability and quality of the images produced by the printer, these printheads are more costly, larger and, therefore, require more space in a printer than those printheads without auxiliary jets.